1. Field of the Invention
The present invention relates generally to a catalytic method for inhibiting the plasma polymerization of methane. More particularly, the present invention relates to the use of a palladium catalyst on a titania substrate in a Raman laser cell comprising methane, to inhibit the formation of deposits on the windows of the Raman cell. The present invention also relates to an improved Raman laser cell incorporating such a catalytic composite.
2. Description of Related Art
Raman laser cells are often used in laser systems to shift the frequency of the laser output beam from one wavelength to another. The Raman laser cell contains a selected gaseous scattering medium with which the pump beam interacts and is thereby converted to a second beam of a different wavelength. The structure and function of such Raman laser cells are well known in the art and will not be detailed here. Because of its high Raman scattering coefficient, methane is a useful gaseous medium for Raman cells. However, it has been found that methane decomposes when subjected to the high intensity beam of the pump laser and leads to the formation of deposits on the cell windows which degrade cell performance.
One approach to this problem is described in U.S. Pat. No. 4,751,714 to Da-Wun Chen, where the difficulty is attributed to the spontaneous decomposition of methane into carbon and hydrogen when subjected to a high intensity beam. The carbon formed then deposits on the inside of the transparent windows of the Raman cell and degrades transmission through these windows. Chen added free gaseous hydrogen to the cell in an attempt to maintain the decomposition and recombination reactions for methane in equilibrium and thus prevent the deposition of carbon particles on the cell window. While this method may lessen the problem with regard to carbon deposits, it does not prevent the build-up of organic deposits on the windows, and thus does not provide a completely satisfactory solution.
Thus, a present need exists to provide an effective method for preventing the formation of deposits on the optical windows of a Raman cell using methane gas as the scattering medium. There is also a need for an improved Raman cell in which deposits are not formed on the optical windows.